Handle constant expressions that have not been constant folded.

Change-Id: I7dd1e6db9a4cee64cb10fb27373d77038b2af63e
Reviewed-on: https://swiftshader-review.googlesource.com/5078
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>